Methods and apparatuses for checking the authenticity of sheet material, in particular bank notes, by means of ultraviolet light are known from U.S. Pat. No. 5,640,463 or U.S. Pat. No. 5,960,103, for example. Known methods and apparatuses are based on the finding that the paper used for bank notes has special properties because it is produced from cotton, on the one hand, and contains no optical brighteners, on the other hand. Upon irradiation with ultraviolet light there is thus no fluorescence effect, i.e. short-wave, ultraviolet light is not converted to longer-wave, visible light. Moreover, a relatively great portion of ultraviolet light is reflected by the surface of the bank note to be evaluated. Forgeries can therefore be recognized if they are irradiated with ultraviolet light for evaluation and the optical brighteners thereby result in longer-wave, visible light. With certain forgeries it has turned out, however, that there is no conversion of short-wave, ultraviolet light to longer-wave, visible light. In these cases a forgery can be recognized, however, when the ultraviolet light reflected by the forgery is examined. If the amount of reflected ultraviolet light should fall below a first, lower threshold value or exceed a second, upper threshold value, a forgery is likewise present, since the amount of reflected ultraviolet light is normally only within a certain range with authentic bank notes.
Known methods and apparatuses for checking the authenticity of sheet material, in particular bank notes, by means of ultraviolet light have the disadvantage, however, that forgeries cannot be recognized if they have no fluorescence effect and the evaluation of the amount of reflected ultraviolet light also fails to yield a clear statement.